A letter written by Bennet and signed by 14 other senators, all Democrats, calls on Holder to employ the “full power of the Department of Justice to review these voter identification laws and scrutinize their implementation.”

A measure that would have required voters to offer photo ID in Colorado passed the Republican-controlled House, but made no headway in the Senate, which is controlled by Democrats. Colorado Secretary of State Scott Gessler, the state’s chief elections official, supported that measure.

Several other states, such as Wisconsin, South Carolina, Alabama, Texas, Kansas and Tennessee have voter-ID measures.

Bennet wrote he is concerned the measures could “block millions of eligible American voters without addressing any problem commensurate with this kind of restriction on voting rights.”

As many as 11 percent of eligible voters nationwide do not have government-issued ID, Bennet said, noting the percentage is higher among seniors, minorities, low-income voters and students.

“Highly restrictive photo identification requirements at the polls can make it more difficult for well-intentioned voters to cast their ballots, and as far as America’s civil rights trajectory is concerned, that sort of effect takes America in the wrong direction,” Bennet wrote.